Allow backward probability model update from tx_size=0

Replace enum items with range definitions.

Change-Id: Iba2b7cac657db5fb6177cb5c9e6f40ec0125d926

av1/common/entropy.c

diff --git a/av1/common/entropy.c b/av1/common/entropy.c
index 870632d..f58f134 100644
--- a/av1/common/entropy.c
+++ b/av1/common/entropy.c
@@ -2913,7 +2913,7 @@
     count_sat = COEF_COUNT_SAT;
   }
 #endif  // CONFIG_ENTROPY
-  for (tx_size = TX_4X4; tx_size <= TX_32X32; tx_size++)
+  for (tx_size = 0; tx_size < TX_SIZES; tx_size++)
     adapt_coef_probs(cm, tx_size, count_sat, update_factor);
 #if CONFIG_RANS
   av1_coef_pareto_cdfs(cm->fc);